Neisseria meningitidis is a causative agent of bacterial meningitis and sepsis. Meningococci are divided into serological groups based on the immunological characteristics of capsular and cell wall antigens. Currently recognized serogroups include A, B, C, D, W-135, X, Y, Z and 29E. The polysaccharides responsible for the serogroup specificity have been purified from several of these groups, including A, B, C, D, W-135 and Y.
N. meningitidis serogroup B ("MenB") accounts for approximately 50 percent of bacterial meningitis in infants and children residing in the U.S. and Europe.
The organism also causes fatal sepsis in young adults. In adolescents, experimental MenB vaccines consisting of outer membrane protein (OMP) vesicles have been found to be approximately 50% protective. However, no protection has been observed in vaccinated infants and children, the age groups at greatest risk of disease. Additionally, OMP vaccines are serotype- and subtype-specific, and the dominant MenB strains are subject to both geographic and temporal variation, limiting the usefulness of such vaccines.
Effective capsular polysaccharide-based vaccines have been developed against meningococcal disease caused by serogroups A, C, Y and W135. However, similar attempts to develop a MenB polysaccharide vaccine have failed due to the poor immunogenicity of the capsular MenB polysaccharide (termed "MenB PS" herein). MenB PS is a homopolymer of (N-acetyl (a2.fwdarw.8) neuraminic acid. Escherichia coli K1 has the identical capsular polysaccharide. Antibodies elicited by MenB PS cross-react with host polysialic acid (PSA). PSA is abundantly expressed in fetal and newborn tissue, especially on neural cell adhesion molecules ("NCAMs") found in brain tissue. PSA is also found to a lesser extent in adult tissues including in kidney, heart and the olfactory nerve. Thus, most anti-MenB PS antibodies are also autoantibodies. Such antibodies therefore have the potential to adversely affect fetal development, or to lead to autoimmune disease.
MenB PS derivatives have been prepared in an attempt to circumvent the poor immunogenicity of MenB PS. For example, C.sub.3 -C.sub.8 N-acyl-substituted MenB PS derivatives have been described. See, EP Publication No. 504,202 B, to Jennings et al. Similarly, U.S. Pat. No. 4,727,136 to Jennings et al. describes an N-propionylated MenB PS molecule, termed "NPr-MenB PS" herein. Mice immunized with NPr-MenB PS glycoconjugates were reported to elicit high titers of IgG antibodies. Jennings et al. (1986) J. Immunol. 137:1708. In rabbits, two distinct populations of antibodies, purportedly associated with two different epitopes, one shared by native MenB PS and one unshared, were produced using the derivative. Bactericidal activity was found in the antibody population that did not cross react with MenB PS. Jennings et al. (1987) J. Exp. Med. 165:1207. The identity of the bacterial surface epitope(s) reacting with the protective antibodies elicited by this conjugate remains unknown.
Peptides can serve as mimics of polysaccharides by binding to polysaccharide-specific antibodies as well as to other polysaccharide binding proteins. For example, concanavalin A (Con A), which binds to oligosaccharides bearing terminal alpha-linked mannose or glucose residues, has been used to select peptide mimetics from random libraries of bacterial phage bearing short peptide sequences at the amino-terminus of the pIII coat protein. oldenberg et al. (1992) Proc. Natl. Acad. Sci. USA 89:5393; Scott et al. (1992) Proc. Natl. Acad. Sci. USA 89:5398. Similarly, monoclonal antibodies have identified peptide mimetics of a carbohydrate present on the surface of adenocarcinoma cells from a phage library. Hoess et al. (1993) Gene 128:43.
Peptides can also elicit polysaccharide-specific antibodies. For example, Westerink et al. (1988) Infect. Immun. 56:1120, used a monoclonal antibody to the N. meningitidis serogroup C ("MenC") capsular polysaccharide to elicit an anti-idiotype antibody. Mice immunized with the anti-idiotype antibody were protected against infection with a lethal dose of MenC bacteria. These experimenters subsequently demonstrated that a peptide fragment of a MenC anti-idiotype antibody elicited serum anti-MenC antibodies and protected animals from bacteremia and death after lethal challenge with MenC bacteria. Westerink et al. (1995) Proc. Natl. Acad. Sci. USA 92:4021.
However, to date, no such approach has been taken with respect to MenB vaccine development. It is readily apparent that the production of a safe and effective vaccine against MenB would be particularly desirable.